Integrated circuit having a parasitic resonance filter

ABSTRACT

A packaged integrated circuit configured for interconnection to an external component comprises a die (1) having a high frequency contact (8), the die (1) being disposed on a lead frame (3). The lead frame (3) comprises a plurality of leads (9). At least two of the leads are first and second RF port leads (9a, 9b) which are electrically connected to the RF port. When mounted to a printed circuit board substrate, there is a capacitor (12) connected between the first and second high frequency contact (8) leads (9a, 9b) to achieve frequency specific signal attenuation at an unwanted frequency with minimal contribution of insertion loss at a desired frequency.

FIELD OF THE INVENTION

The present invention relates to integrated circuits and moreparticularly to a high frequency integrated circuit having filteredports.

BACKGROUND OF THE INVENTION

High frequency systems, including radio frequency (RF) and microwavesystems, generally comprise multiple high frequency integrated circuits(IC) and discrete devices interconnected on a printed circuit board.Typically, a high frequency system requires generation and amplificationof a distortion free fundamental operating frequency. As a practicalmatter, high frequency components and systems often generate unwantedharmonics of the fundamental operating frequency through amplificationand switching functions conventionally performed within the system. Itis desirable to filter out these harmonics generated by the system sothat the signal with which the system operates is as free of harmonicdistortion as possible. A low harmonic distortion system generallyexhibits better efficiency overall. In order to reduce harmonicdistortion, it is conventional to filter out harmonics of thefundamental frequency at an RF output port on one or more of the ICswithin the system. It is a matter of designer judgment at which IC orICs to place the filter.

An article entitled "Minimizing GSM Mobile-Terminal Design Risk AllowsEasier Upgrades" by Kalinka & Baker from the May 1996 publication ofWireless Systems Design magazine, shows conventional filtering ofharmonic distortion at both the input of a power amplifier (PA) and theoutput of a low noise amplifier (LNA)/RF mixer in a GSM terminal. Thefilters shown by way of block diagram representation typically comprisefrom three to seven discrete elements or ceramic resonators soldered tothe printed circuit board and connected to an RF port lead of the IC.Disadvantageously, these filters can be complex and generally consume arelatively large amount of usable surface area on the printed wiringboard. Additionally these filters can contribute up to 1 dB of insertionloss to the system. In the case of a power amplifier IC used withconventional filtering, in order to achieve the desired system outputpower, the power amplifier must output sufficient power to supply theoutput power required by the system at the output stage of the filter inaddition to the power lost due to the insertion loss of the filter. Thisadditional output power requires more DC current be drawn by the poweramplifier. The added DC current drain disadvantageously decreasesefficiency and increases heat generation. Accordingly, the traditionalfiltering contributes to a larger, more complex, and less efficientsystem design which is in contravention of the interest to miniaturizeand to extend battery life.

There is a need, therefore, for improved filtering in a high frequencysystem that does not adversely effect insertion loss, efficiency orminiaturization.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve filtering in a highfrequency system.

It is an object of the present invention to provide filtering for an ICwithout increasing insertion loss.

It is another object of the present invention to reduce external filtercomplexity.

It is another object of the present invention to reduce the printedcircuit board surface area traditionally required for external filteringof high frequency ICs.

It is another object of the present invention to decrease the harmonicdistortion present in a high frequency system.

A packaged IC configured to be connected to an external filter comprisesa die having an RF port. The die is disposed on a lead frame whichcomprises a plurality of leads. At least two of the leads are first andsecond RF port leads. The first and second RF port leads areelectrically connected to the RF port.

It is an advantage of an IC according to the teachings of the presentinvention that an undesired signal may be filtered without adverselyeffecting insertion loss, efficiency, or miniaturization.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example andin conjunction with the following figures in which:

FIG. 1 is a functional schematic of a known transmit/receive diversityswitch.

FIG. 2 is a known pinout of a packaged diversity switch.

FIG. 3 is a functional schematic of a transmit receive diversity switchaccording to the teachings of the present invention.

FIG. 4 is a pinout of a packaged diversity switch according to theteachings of the present invention.

FIG. 5 is a schematic diagram of an equivalent circuit of a filteraccording to the teachings of the present invention.

FIG. 6 is a graph of output power relative to input power versusfrequency for an integrated circuit without filtering.

FIG. 7 is a graph of output power relative to input power at a filteredRF port of an integrated circuit having filtering according to theteachings of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of describing a preferred embodiment of the presentinvention, there is shown in the drawings an implementation of a GaAsbased transmit/receive diversity switch according to the prior art andaccording to the teachings of the present invention. A functionalschematic of a prior art transmit/receive diversity switch is shown inFIG. 1 of the drawings. As one of ordinary skill in the art canappreciate, the switch is made up of a symmetrical die. FIG. 2 of thedrawings illustrates the prior art pin out of the diversity switch whichshows the die (1) epoxied to a die attach paddle (2) of a SSOP-8 leadframe (3). The standard SSOP-8 lead frame (3) has a total of eightleads. A plastic body (15) molded around the lead frame (3), is shown inphantom line representation. A ceramic or glass package body (not shown)is also appropriate as are other types and sizes of lead framesincluding the MSOP,SOIC, TSSOP and SOT packages. The die (1) hascontacts (5) thereon providing appropriate electrical access to and fromthe die (1). Each contact (5) is wire bonded according to conventionalprocedure to appropriate leads of the lead frame (3).

The diversity switch packaged in an eight lead SSOP is shown configuredfor filtering a first RF port (8). In the prior art pin out as shown inFIG. 2, a high frequency contact provides access to an RF port and isinterconnected to a single RF port lead (9). In an embodiment of thediversity switch according to the teachings of the present invention andwith specific reference to FIGS. 3 and 4 of the drawings, adjacent highfrequency contacts (8) are electrically identical and provide access tothe RF port. Each high frequency contact is wirebonded to two adjacentleads (9a, 9b). The pin out further comprises two DC bias leads (6), oneground lead (7), and three RF ports each connected to one lead (9) each.The specific wire bond configuration depends upon the number of leadsavailable in the package, the number of DC biases and grounds necessaryfor the IC (1), and the desired filtering for the RF ports. In theembodiment shown in FIG. 4 of the drawings, the wire bond configurationdeviates from the conventional configuration of each contact (5) wirebonded to a single lead, in that the RF output port to be filtered hastwo wire bonds (10a, 10b) electrically interconnecting a single highfrequency signal contact (8) to two signal leads (9a, 9b). A first wirebond (10a) is connected to a first RF port lead (9a) and a second wirebond (10b) is connected to a second RF port lead (9b). In a lesspreferred embodiment, the IC presents the same RF port (8) signal at twodifferent high frequency contacts (5) on the die (1).

In that case, each RF port lead (9a, 9b) is electrically connected to adifferent high frequency each different contact (5) signal contactscarrying substantially the same signal. When the packaged IC isassembled and soldered to a printed wiring board substrate, each RFoutput port lead (9a, 9b) is electrically connected to a length ofconductive trace, which at RF and microwave frequencies, behaves as atransmission line. First RF output port lead (9a) is connected to firsttrace (11a) and second RF output port lead (9b) is connected to secondtrace (11b) in connecting elements comprising known manner.

It is an object of the present invention to take advantage of theparasitic inductance already present in the wirebond (10a, 10b), lead(9a, 9b) and transmission line (11a, 11b) and use it to create afiltering circuit for use in conjunction with the IC (1).

Pursuant to the objective, a capacitor (12) is connected between thefirst and second traces (11a, 11b). The parasitic inductance associatedwith each wire bond (10a, 10b), RF port lead (9a, 9b) and trace (11a,11b) when interconnected with capacitor (12) creates a filter circuit. Alumped element equivalent circuit of the parasitic resonance filtercircuit is shown in FIG. 5 of the drawings. This filter circuit is tunedby appropriate estimate or measurement of the inductance of the wirebond (10a), lead (9a), and trace (11a) and by appropriate selection ofthe value of the capacitor (12) to resonate at a desired frequency.Typically, the highest power unwanted spectral component of anRF/microwave system is the second harmonic of the fundamental operatingfrequency. Accordingly, for a 900 Mhz system, the capacitor (12) isselected to create a 1800 Mhz resonant circuit.

Each series combination of the wire bond (10a, 10b), the lead (9a, 9b),and the trace (11a, 11b) is equivalently represented as an inductor. Asthere are two similar combinations, each is represented by inductivevalues L1 and L2, respectively, having substantially equal value. Theresonant frequency of the equivalent circuit occurs when the admittance(Y) of the filter circuit is zero. Accordingly, the admittance Y_(L1) ofthe inductor (10a, 9a, 11a) represented by L1 when summed with theadmittance (Y_(L2),C) of the series inductor (10b, 9b, 11b) representedby L2 and parallel capacitor (12) represented by C must equal zero:

    Y.sub.L1 +Y.sub.L2,C1 =0

Representing the admittance in terms of frequency and inductance:##EQU1## Because the value of L₁ equals L₂, the foregoing equation maybe simplified by representing L₁ and L₂ with the single inductive valueL where: ##EQU2## Where f is the 2nd harmonic frequency.

The appropriate selection of the value of capacitor (12) for a givenfrequency, therefore, is a function of the parasitic inductance (L1,L2). Advantageously, only a single external component is required forimplementation of the parasitic resonance filter and the space withinwhich the external capacitor physically fits is significantly less thanthe space required for implementation of traditional filtering havingmore lumped elements. Space minimization is of particular importance inportable applications. Also of importance is that the parasiticresonance filter does not contribute insertion loss to the system at theoperating frequency.

In an SSOP-8 IC package, the inductance of the wire bond (10a, 10b), thelead (9a, 9b), and the trace (11a, 11b) may be approximated to be 3 nH.This assumes use of gold wire bonds of 1 mil diameter, and 200 mil 50ohm traces connecting to the external capacitor (12). This also assumesthat the packaged IC is mounted to a printed circuit board substrateaccording to Application Note Number M538 entitled "Surface MountingInstructions" published by M/A-COM, Inc. the contents of which arehereby incorporated by reference. For a 3 nH inductance, the appropriatevalue of the external capacitor (12) to create a parasitic filter thatresonates at 1.8 Ghz is: ##EQU3##

The performance of the known RF transmit/receive diversity switchwithout filtering is shown in FIG. 6 of the drawings. The insertion lossmeasurement was taken at the RF port lead. FIG. 7 of the drawings showsthe performance of the same switch using a parasitic resonance filteraccording to the teachings of the present invention. The insertion lossmeasurement was taken at a filtered RF port (14). The particular filterimplemented is designed to resonate at a second harmonic frequency of1800 Mhz for a 900 Mhz system. As shown, there is minimal relativesignal degradation at the desired fundamental operating frequency and astrong attenuation at the undesired second harmonic frequency. The sharpresonance seen at the second harmonic is the result of high Q factors ofthe parasitic inductors and the capacitor used to achieve the filtering.In this example, approximately 12 dB of rejection is achieved at thesecond harmonic frequency. Advantageously, signal degradation due toinsertion loss is not increased by the addition of the capacitor (12).Because the package interconnects are inescapably present in anypackaged IC mounted on a printed circuit board substrate and inasmuch asthe parasitic inductances are inescapably present in the package,wirebonds and traces, filtering according to the teachings of thepresent invention contributes very little insertion loss at thefundamental operating frequency while performing the desirable functionof second harmonic attenuation.

Filtering according to the teachings of the present invention is shownspecifically for a diversity switch. The techniques of utilizing thewirebond, lead, and trace inductance, however, has significantadvantages when utilized in conjunction with other high frequency ICs.As a specific example, a power amplifier integrated circuit having afiltered RF port according to the teachings of the present invention canbe operated at a lower DC operating point. Consequently, there is lesscurrent drain than in the system which uses traditional filteringleading to lower operating temperatures and longer battery life.

Additionally, filtering according to the teachings of the presentinvention may also be implemented at RF input ports in order to filterundesired harmonics prior to operation on the signal by the IC. Morethan one filter according to the teachings of the present invention maybe implemented on a single IC provided there are a sufficient number ofleads available to dedicate two leads to each RF port to be filtered. Asan example, in the embodiment shown for a receive/transmit diversityswitch, both the Tx and the ANT1 RF ports may be filtered by dedicatingthe two GND leads (6) to the second RF port lead of the TX and ANT1 RFports respectively. In this example, the switch may still be packaged inan SSOP-8 package because this particular switch operates in a floatingcondition. If the GND leads (6) were necessary, the same switch could bepackaged in a larger package having additional available leads tosupport filtering of more than one RF port.

Other advantages of the invention are present from the detaileddescription by way of example and from accompanying drawings, and fromthe spirit and scope of the appended claims.

We claim:
 1. An IC package comprising a die having an RF port, the diebeing disposed on a lead frame comprising a plurality of leads, one ofsaid leads being a first RF port lead which is electrically connected tosaid RF port, said first RF port lead being connected to an externalcomponent,a second RF port lead, electrically connected to said RF port,and an external capacitor connected between said first and second RFport leads.
 2. An IC package as recited in claim 1 wherein said firstand second RF port leads are adjacent each other.
 3. An IC package asrecited in claim 1 wherein the package is ceramic.
 4. An IC package asrecited in claim 1 wherein the package is plastic.
 5. An IC package asrecited in claim 1 wherein the package is glass.
 6. An IC package asrecited in claim 1 wherein said RF port further comprises a plurality ofhigh frequency contacts carrying substantially the same signal and saidfirst RF port lead is connected to one of said plurality of highfrequency contacts and said second RF port lead is connected to anotherone of said plurality of high frequency signal contacts.
 7. An IC as acomponent part of a circuit comprising:a die having a high frequencycontact, and being connected to the circuit through first and secondconnecting elements a filter electrically connected to said highfrequency contact the filter comprising a first inductive impedanceelectrically disposed in parallel with a second inductive impedance inseries with a capacitive impedance, wherein said first and secondinductive impedances comprise the first and second connecting elements.8. An IC as recited in claim 7 wherein said capacitive impedance has avalue that is chosen to resonate with said first and second inductiveimpedances.
 9. An IC as recited in claim 7 wherein said first and secondinductive impedances are substantially similar.
 10. An IC as recited inclaim 8 wherein said first and second inductive impedances aresubstantially similar.
 11. An IC as recited in claim 7 wherein saidcapacitive impedance comprises a capacitor external to the IC.
 12. An ICas recited in claim 7 wherein each said connecting element extends fromsaid high frequency contact.
 13. An IC as recited in claim 7 whereinsaid high frequency contact is a first high frequency contact andfurther comprising a second high frequency contact, wherein said firstand second high frequency contacts carry substantially the same signal.14. An IC as recited in claim 13 wherein said first and second highfrequency contacts are adjacent each other.